1. Field of the Invention
This invention relates to an acidic bleaching solution that generates little or no chlorine gas during use and which is particularly effective as a cleaner for removing soapscum, limescale, mold and mildew from treated surfaces. The invention also relates to a bleaching system for forming the acidic bleaching solution and a method of preparing the acidic bleaching solution.
2. Related Background Art
Bleach/sulfamic acid cleaning compositions have long been known. For example, UK Patent Application GB 932,750 discloses a powdered cleansing composition containing alkali metal monopersulfate salts and alkali metal chlorides in combination with a nitrogen-containing chlorine-hypochlorite acceptor such as sulfamic acid. The chlorine generated upon the addition of water to the composition is said to be tied up by the nitrogen-containing chlorine-hypochlorite acceptor so as to reduce or eliminate the expected chlorine odor.
A sanitizing composition which is said to have an improved shelf life in the dry state is described in UK Patent Application GB 2078522. The composition comprises sodium or calcium hypochlorite, an acid source which desirably includes sulfamic acid in combination with another non-reducing acid such as malic acid or succinic acid and a surfactant. The acid content of the composition is said to enhance the ability of the composition to sanitize surfaces with lime scale or milk stone. This composition, however, has been reported to evolve chlorine gas when stored in damp conditions or when prepared in concentrated aqueous solutions.
U.S. Pat. No. 4,822,512 reportedly overcomes this problem through the use of a low level of water-soluble inorganic halide in the composition, such as sodium chloride. In particular, a water-soluble biocidal composition is described as (a) 0.01 to 5 parts by weight of a water-soluble inorganic halide, (b) 25 to 60 parts by weight of an oxidizing agent which, in aqueous solution, reacts with halide to generate hypothalite ions, (c) 3 to 8 parts by weight of sulfamic acid, (d) 0 to 20 parts by weight of an anhydrous non-reducing organic acid such as malic acid or succinic acid and (e) 10 to 30 parts by weight of an anhydrous alkali metal phosphate. The pH of a 1% by weight aqueous solution of this composition is between about 1.2 and 5.5. The aforementioned compositions, however, are directed to dry or powder compositions and thus do not contemplate the problems associated with aqueous liquid bleach solutions.
In particular, it is well known that the addition of an aqueous hypochlorite solution to an acidic cleaning solution will generally result in the evolution of potentially dangerous amounts of chlorine gas. A number of compositions have been proposed in an attempt to overcome this problem. U.S. Pat. No. 3,749,672 is directed to buffered aqueous solutions having a pH between 4 and 11 which are prepared by adding a hypochlorite such as sodium hypochlorite to certain N-hydrogen compounds such as sulfamic acid. In particular, it is said that stable bleaching compositions under acid conditions (e.g. pH of about 4.0 to 6.9) may be obtained when there is an excess of sulfamate (e.g., a mole ratio less than 2:1 of hypochlorite to sulfamate). However, the presence of the excess sulfamate is likely to result in a hypochlorite-sulfamate complex which will decrease the bleaching kinetics or efficiency of the composition.
U.S. Pat. No. 5,503,768 describes a halogen scavanger constituted by an aromatic ring and at least one group which contains a lone-pair-containing hetero atom adjacent to the aromatic ring. The electron donating aromatic compound, i.e., the halogen scavenger, can be added to an acid cleaner which when mixed with an oxidizing agent such as sodium hypochlorite prior to use suppresses the release of halogen gas. It is reported that it is desirable to add the electron donating aromatic compound to the acid cleaner in an approximately equal molar amount to the halogen estimated to be released upon the mixture of the acid cleaner with the oxidizing agent.
There continues, however, to be a need for acidic bleaching compositions that do not result in the substantial generation of potentially hazardous chlorine gas when a hypochlorite solution is mixed with an acidic solution. Such acidic bleaching compositions, i.e., low chlorine gas generation, that have excellent bleaching efficacy are particularly desirable.